WASTEWATER
Design and construction of an innovative effluent outfall in the challenging Muskwa River By Jurek Janota-Bzowski
I
t is not often in one’s career that a project so obviously demands a high level of creative freedom in designing a large river outfall for disinfected effluent from a municipal treatment plant. Such was the case with the Northern Rockies Regional Municipality (NRRM) who contracted Kerr Wood Leidal Associates to design a 25 ML/d river outfall in the Muskwa River for the Town of Fort Nelson, British Columbia. Initially, this seemed like a dream opportunity to build something special, design some unique features, and incorporate some innovative ideas that had never been 12 | August 2019
designed before. However, once the initial euphoria had passed, the magnitude of the task ahead became readily apparent. Challenges and constraints for operation, maintenance and construction started to seem overwhelming. These included: • Designing, operating, and monitoring an outfall in a very active river that ranges in depth from 0.75 m (half of which is ice) in winter, to 8 m in summer. • Mitigating the risk for an outfall in a fast-flowing river, where the key objectives are to: protect the manifold and diffusers from extensive river scour; withstand impact forces from debris, logs, and ice breakup; and ensure diffus-
ers are flexible, self-cleaning, and operating within a 0.3 m water zone beneath the ice in winter. • Making provisions for an outfall to survive the loss of any diffuser without blocking the manifold through ingress of sands and gravel. • Building an outfall with its manifold 3 m below the existing riverbed level. • Incorporating operation and maintenance features that allow operators to: inspect and monitor performance of each diffuser and replace as necessary; clean out the entire length of the manifold using suction, air, or water pressure; and re-excavate the manifold in case of catastrophic failure at minimum cost.
Environmental Science & Engineering Magazine
